The present invention relates to a circuit configuration for the compensation of leakage currents in a voltage-controlled oscillator of a PLL circuit.
In modern PLL circuits, the voltage-controlled oscillator is set to the required frequency value by means of a control voltage which is generated in that the output current of a charge pump charges a capacitor within a loop filter, which precedes the voltage-controlled oscillator. The output current of the charge pump, in its turn, is directly proportional to the output signal of the phase detector, normally being available within the PLL circuit. The varicap diodes normally used as frequency-determining elements in the oscillator have a temperature-dependent leakage current, which causes a constant discharge of the capacitor in the loop filter. This results in a constant phase error at the oscillator output, since the control voltage applied to the oscillator is no longer proportional to the output signal of the phase detector, but is now modified by the leakage current. Especially at high temperatures, this leakage current can assume high values, even to the extent that the PLL circuit is no longer able to remain in a latched state.
The invention provides a circuit with an arrangement in such a way that the leakage current originating from the varicap diodes can no longer exert a negative influence on the control behavior of the PLL circuit.
According to the invention, a compensation circuit is provided which contains varicap diodes in the same configuration as in the oscillator, that in the compensation circuit an operational amplifier with a differential output, connected as voltage follower, is provided which has an input connected to the control line, and which at one of its outputs reproduces the control voltage and applies it to the varicap diodes in the compensation circuit, and which at its other output supplies a current that is equal to the leakage current of the varicap diodes in the compensation circuit, and that a current shunt-off path is connected to the control line which allows a current to drain off from the control bus which corresponds to the current supplied by the operational amplifier.
In the circuit arrangement according to the invention, a current is generated which corresponds exactly to the leakage current of the varicap diodes flowing through the voltage-controlled oscillator. This current is used to compensate the leakage current, so that it can no longer cause any changes in the control voltage applied to the oscillator. The circuit, therefore, ensures stable operation of the PLL circuit over a large temperature range.
A preferred further development of the invention provides for the current shunt-off path to be the output branch of a current mirror circuit, to whose input branch is applied the current output by the operational amplifier. This embodiment allows the desired compensation of the leakage current by simple means.